1. Field of the Invention
The present invention relates to an optical fiber apparatus provided with a demultiplexing/multiplexing unit on its fiber's end surface, such as an optical fiber provided with an optical detector array having a demultiplexing function for separating wavelength multiplexed signals of a wavelength division multiplexing (WDM) optical transmission and for receiving separated optical signals by respective optical detectors, an optical detecting apparatus provided with a demultiplexing-multiplexing unit on its light receiving surface, and an optical transmission system using the same.
2. Related Background Art
Conventionally, the following combination structures were proposed as an optical detector having a demultiplexing function for demultiplexing wavelength multiplexed signals and for receiving respective demultiplexed optical signals, as disclosed in Japanese Laid-Open Patent Nos. 8(1996)-82711 and 8(1996)-211237. In one structure, a device having a combination of a branching function and an optical filter (band-pass) function is used. In another structure, so-called array-waveguide diffraction gratings are used as a demultiplexer and an optical detector (or an optical detector array) is combined with the diffraction gratings. In the array-waveguide diffraction gratings, a plurality waveguides having different optical lengths are combined, and different wavelengths are coupled to different output waveguides due to an interference effect.
The prior art device of Japanese Laid-Open Patent No. 8(1996)-82711, however, has the following technical disadvantages since optical separation is effected using multiple reflections:
1. External light must be collimated and then input; PA1 2. The angle of input light must be set to a given angle; and PA1 3. Performance of the optical filter must be varied according to its location. PA1 1. An optical fiber must be optically coupled to the waveguide; PA1 2. A possibility exists that variation in a waveguide width of the array-waveguide gratings adversely affects characteristics; PA1 3. A thin waveguide device must be fixed to a fiber end; and PA1 4. A slab waveguide portion must be precisely processed to uniformly distribute light.
Further, the prior art device of Japanese Laid-Open Patent No. 8(1996)-211237 has the following technical disadvantages since demultiplexing is effected using interference between different optical lengths:
In addition to those disadvantages, in both prior art devices, an optical coupling system is needed between the optical fiber and the demultiplexer, and a portion having a separating function (spatial separation) and a size larger than the optical fiber is needed.
Furthermore, there is not yet provided an optical fiber (typically, a plastic optical fiber having a relatively large core diameter) which has a demultiplexing/multiplexing unit provided directly on its end portion to solve the above disadvantages.